1. Field of the Invention
The present invention relates to a method and device for the calibration of MIMO systems.
2. Description of the Related Technology
In the following description multiple-input multiple output (MIMO) systems are considered, i.e. multiple antenna wireless communication systems with multiple antennas at both sides of the link.
FIG. 1 shows the considered application context, namely for single-user MIMO downlink. When implementing such systems in practice, system designers are very much concerned with analogue front-end impairments. The complete channel to be taken into account is composed not only of the propagation channel, but also of the multiple antenna analogue transceiver front-ends. As an example, FIG. 2 shows some non-ideal effects in a multi-antenna transmitter front-end that can have a detrimental impact on the end-to-end performance. The gain/phase mismatch between multiple antenna transmitters (TXs) or multiple antenna receivers (RXs) is a MIMO specific analogue front-end impairment.
Downlink channel knowledge (channel state information (CSI)) is required at the access point (AP) transmitter both for MIMO with transmit processing and MIMO with joint transmit/receive processing. For time-division-duplex (TDD) wireless systems, one way to acquire this downlink channel knowledge is to apply a so-called feedback approach: the AP transmits a known preamble to the user terminal, the downlink CSI is extracted at the user terminal side and subsequently retransmitted to the access point over the uplink. It is clear this approach involves a big overhead, which kills the capacity gain.
An alternative is the reciprocal approach, where the transpose of the estimated uplink CSI is used as an approximation of the downlink CSI and applied into the transmit processing. The reciprocal approach minimizes the overhead caused by acquiring the downlink CSI in the AP. For a reciprocal approach, the received signal vector {circumflex over (x)} at the user terminal (UT) can be written as (for transmit Zero-Forcing)
                              x          ^                =                                                                              D                                      RX                    ,                    UT                                                  ⁢                                  HD                                      TX                    ,                    AP                                                                              ︸                                  H                  DL                                                      ·                                                            D                                      RX                    ,                    AP                                                        -                    1                                                  ⁢                                  H                                      -                    1                                                  ⁢                                  D                                      TX                    ,                    UT                                                        -                    1                                                                              ︸                                  F                  =                                                            (                                              H                        UL                                            )                                                              -                      T                                                                                            ·            x                    +          n                                    (                  eq          .                                          ⁢          1                )            where H denotes the DL propagation channel and n the additive white Gaussian noise (AWGN). The diagonal components in diagonal matrix DTX,AP and DRX,AP are the transfer functions of the transmit (Tx) and receive (Rx) front-ends (FEs) at the access point (AP). To recover the data, the reciprocity of the AP analogue FEs is required, which is equivalent to
                                          D                          TX              ,              AP                                ·                      D                          RX              ,              AP                                      -              1                                      =                              [                                                                                                      TX                      1                                                              RX                      1                                                                                        0                                                  …                                                  0                                                                              0                                                                                            TX                      2                                                              RX                      2                                                                                        …                                                  0                                                                              ⋮                                                  ⋮                                                  …                                                  ⋮                                                                              0                                                  0                                                  …                                                                                            TX                      nT                                                              RX                      nT                                                                                            ]                    =                      ξ            ·            I                                              (                  eq          .                                          ⁢          2                )            in which ξ is a coefficient. The reciprocity requirement can be translated into
                                          TX            1                                RX            1                          =                                            TX              2                                      RX              2                                =                      …            =                                                            TX                  nT                                                  RX                  nT                                            =              ξ                                                          (                  eq          .                                          ⁢          3                )            When there is AP Tx and/or Rx mismatch, the reciprocity of the complete channel is destroyed, which results in multi-stream-interference (MSI) and causes severe performance degradation. Hence, a calibration of the analogue front-ends of the MIMO system is needed to ensure the complete channel reciprocity. A calibration should yield coefficients cj of a calibrating matrix C, for which the calibration requirement can be written as
                                          c            1                    ·                                    TX              1                                      RX              1                                      =                                            c              2                        ·                                          TX                2                                            RX                2                                              =                      …            =                                                            c                                      n                    T                                                  ·                                                      TX                                          n                      T                                                                            RX                                          n                      T                                                                                  =              α                                                          (                  eq          .                                          ⁢          4                )            For matrix C one can write:
                    C        =                              [                                                                                c                    1                                                                    …                                                  0                                                                              ⋮                                                  ⋱                                                  ⋮                                                                              0                                                  …                                                                      c                    nT                                                                        ]                    =                      [                                                                                                                              RX                        1                                                                    TX                        1                                                              ·                    α                                                                    …                                                  0                                                                              ⋮                                                  ⋱                                                  ⋮                                                                              0                                                  …                                                                                                                    RX                        nT                                                                    TX                        nT                                                              ·                    α                                                                        ]                                              (                  eq          .                                          ⁢          5                )            
In the paper ‘OFDM-MIMO WLAN AP Front-end Gain and Phase Mismatch Calibration’ J. Liu et al., Proc. IEEE RAWCON, September 2004, the calibration scheme as shown in FIG. 3 was presented. The scheme calibrates access point transceiver FE mismatches by using a pair of calibration TX and RX, plus the power directional couplers and the power combiner/divider on the calibration board. Although by implementation the scheme has proven to work in a stable and effective way, it suffers from the main drawback that it is not cost-effective from an implementation point of view, since it requires a calibration transceiver.
Another prior art calibration scheme is presented in WO2004/039022. The calibration is performed ‘over the air’, i.e. the complete channel transfer functions HAP->UT and HUT->AP are determined. Subsequently the following matrix formula is solved in order to derive the coefficients KA and KB.ĤUT->AP·KA=(ĤAP->UT·KB)T  (eq.5)Although no additional calibration hardware is needed, the calibration factor measurement involves a considerable overhead. In case the calibration needs to be redone frequently, the overhead will reduce the capacity.
In European patent application EP1392004-A2 a calibration method is disclosed for a wireless communication transceiver comprising at least one transmitter/receiver pair wherein front-end mismatches occur. The method requires the use of amongst other things a splitter, a directional coupler, a calibration noise source and a power splitter. The method introduces some matching requirement on the calibration hardware, which is to be minimized.
European patent application EP1496567-A1 relates to an arrangement for calibrating transmission and/or reception of signals in a radio communication system. The arrangement comprises a number of transceiving means each connected to an antenna element, a coupling network arranged between the transceiving means and the antenna elements and connected to calibration transmitting and/or receiving means that feed test signals to and/or receive the test signals from the coupling network, a calibration processor for determining variations of the test signals in the transceiving means, and a beamforming processor that takes into account the variations for beamforming and/or determines the arrival direction of transceived radio signals.